The head of BoeingTed Colbert, who heads .’s defense division, is leaving the company effective immediately, CEO Kelly Ortberg said, marking his first major leadership change since taking the top job in early August.

“At this critical juncture, our priority is to restore our customers’ trust and meet the high standards they expect from us to enable their important missions around the world,” Ortberg said in a staff memo Friday. “By working together, we can and will improve our performance and ensure we meet our commitments.”

Ortberg thanked Colbert for his 15 years of service at Boeing and said the unit’s chief operating officer, Steve Parker, will take over until the company names a successor for Colbert.

Boeing’s defense, space and security division generated nearly 40% of Boeing’s revenue in the first half of this year, but struggled with production problems and cost overruns, including on the new 747s that will serve as Air Force One planes. In the space sector, Boeing’s Starliner is returning without the NASA astronauts that carried it to the International Space Station in June. Instead, they will take back SpaceX’s Crew-9 vehicle, NASA said last month.

Colbert did not immediately respond to CNBC’s request for comment.